1. Field of the Invention
The present invention relates in general to the field of signal processing, and, more specifically, to a system and method that includes inductor over-current protection in a switching power converter based on one or more non-inductor-current signals.
2. Description of the Related Art
Switching power converters convert supplied power into a form and magnitude that is useful for numerous electronic products including cellular telephones, computing devices, personal digital assistants, televisions, other switching power converters, and lamps, such as light emitting diode and gas discharge type lamps. For example, alternating current (AC)-to-direct current (DC) switching power converters are often configured to convert AC voltages from an AC voltage source into DC voltages. DC-to-DC switching power converters are often configured to convert DC voltages of one level from a DC voltage source into DC voltages of another level. Switching power converters are available in many types, such as boost-type, buck-type, boost-buck type, and Cúk type converters. The switching power converters are controlled by a controller that controls one or more power regulation switches. Switching of the power regulation switch controls the link voltage of the switching power converter and, in some embodiments, also controls power factor correction.
FIG. 1 represents a power control system 100, which includes a switching power converter 102 and a controller 110. Voltage source 104 supplies an alternating current (AC) input voltage VIN to a full, diode bridge rectifier 106. The rectifier 106 can be separate from the switching power converter 102, as shown, or part of the switching power converter 102. The voltage source 104 is, for example, a public utility, and the AC input voltage VIN is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. The rectifier 106 rectifies the input voltage VIN and supplies a rectified, time-varying, line input voltage VX to the switching power converter.
The switching power converter includes a power regulation switch 108, and the power control system 100 also includes a controller 110 to control power regulation switch 108. Switch 108 is an n-channel, metal oxide semiconductor field effect transistor (FET). In other embodiments, switch 108 is a bipolar junction transistor or an insulated gate bipolar junction transistor. Controller 110 generates a gate drive control signal CS0 to control the switching period and “ON” (conduction) time of switch 108. Controlling the switching period and “ON” time of switch 108 provides power factor correction and regulates the link voltage VLINK. Switch 108 regulates the transfer of energy from the line input voltage VX through inductor 112 to link capacitor 114. The inductor current iL ramps ‘up’ when switch 108 is “ON”, and diode 116 prevents link capacitor 114 from discharging through switch 108. When switch 108 is OFF, diode 116 is forward biased, and the inductor current iL ramps down as the current iL recharges link capacitor 114. The time period during which the inductor current iL ramps down is referred to as an “inductor flyback period”. The switching power converter 102 also includes a low pass, electromagnetic interference (EMI) filter 118 to filter any high frequency signals from the line input voltage VX. The EMI filter 118 consists of inductor 120 and capacitor 122.
Link capacitor 114 supplies stored energy to load 117. Load 117 can be any type of load such as another switching power converter, light source, or any other electronic device. The capacitance of link capacitor 114 is sufficiently large so as to maintain a substantially constant output, link voltage VLINK, as established by controller 110. The link voltage VLINK remains substantially constant during constant load conditions. However, as load conditions change, the link voltage VLINK changes. The controller 110 responds to the changes in link voltage VLINK and adjusts the control signal CS0 to restore a substantially constant link voltage VLINK as quickly as possible.
Controller 110 maintains control of the inductor current iL to ensure safe operation of switching power converter 102. Numerous fault conditions can occur that can cause the inductor current iL to exceed normal operating limitations. For example, ringing in the EMI filter 118 can cause the inductor current iL to exceed normal operating conditions. “Ringing” refers to oscillations of a signal around a nominal value of the signal. Ringing can be associated with sharp (i.e. high frequency component) transitions. To maintain control of the inductor current iL, switching power converter 102 includes an inductor current sense resistor 124 connected in series with switch 108 to sense the inductor current iL. The inductor current iL causes an inductor current signal in the form of inductor current sense voltage ViL—sense to develop across inductor sense resistor 124. The inductor current sense voltage ViL—sense is directly proportional to the inductor current iL when switch 108 is ON. Controller 110 monitors the inductor current sense voltage ViL—sense to determine if inductor current iL exceeds typical operating limitations and responds to an atypically large inductor current iL by deasserting the control signal CS0. Deasserting control signal CS0 causes switch 108 to turn OFF, thereby attempting to prevent any further increase of the inductor current iL.
Controller 110 controls switch 108 and, thus, controls power factor correction and regulates output power of the switching power converter 102. The goal of power factor correction technology is to make the switching power converter 102 appear resistive to the voltage source 104. Thus, controller 110 attempts to control the inductor current iL so that the average inductor current iL is linearly and directly related to the line input voltage VX. Prodić, Compensator Design and Stability Assessment for Fast Voltage Loops of Power Factor Correction Rectifiers, IEEE Transactions on Power Electronics, Vol. 22, No. 5, September 2007, pp. 1719-1729 (referred to herein as “Prodić”), describes an example of controller 110. The controller 110 supplies a pulse width modulated (PWM) control signal CS0 to control the conductivity of switch 108. The values of the pulse width and duty cycle of control signal CS0 generally depend on feedback signals, namely, the line input voltage VX, the link voltage VLINK, and inductor current sense voltage ViL—sense.
FIG. 2 depicts inductor current iL and control signal CS0 timing diagrams 200 during a period TT of switch control signal CS0. Referring to FIGS. 1 and 2, for the time period t1, controller 110 generates a pulse 202 of control signal CS0 that causes switch 108 to conduct. When switch 108 conducts, the inductor current iL ramps up. The time period t1 is the pulse width (PW) of control signal CS0 for period TT of control signal CS0. When the pulse of control signal CS0 ends at the end of time period t1, the inductor current iL begins to ramp down. The inductor current iL ramps down to 0 at the end of time period t2. Time period t2 is an inductor flyback period. The time period t3 represents the elapsed time between (i) the inductor flyback period for period TT and (ii) the next pulse of control signal CS0. To operate switching power converter 102 in discontinuous current mode (DCM), controller 110 ensures that the time period t3 is non-zero. In other words, to operate in DCM, the inductor current iL must ramp down to 0 prior to the next pulse 204 of control signal CS0.
To monitor the inductor current iL when energy is being transferred to the inductor 112 during time t2 (FIG. 2), controller 110 monitors inductor current sense voltage inductor current sense voltage ViL—sense. The inductor current sense voltage ViL—sense provides a direct one-to-one tracking of the inductor current iL when energy is being transferred to the inductor 112. To ensure that switching power converter 102 operates in DCM, switching power converter 102 includes a secondary coil 126 that develops a voltage signal VL corresponding to the inductor current iL. Comparator 128 determines if voltage signal VL is greater than 0V. The comparator 128 generates an output signal FLYBACK. When signal FLYBACK is a logical 0, switching power converter 102 is in an inductor flyback period. When signal FLYBACK is a logical 1, switching power converter 102 is not in an inductor flyback period. A logical “1” is, for example, a 3.3V. Thus, in one embodiment, when signal FLYBACK is a logical 1, a 3.3V signal is applied to terminal 130 of controller 110. Controller 110 receives the signal FLYBACK through terminal 130 and uses the signal FLYBACK to ensure that control signal CS0 does not begin a new pulse 204 until the inductor flyback period is over. Thus, controller 110 is able to maintain switching power converter 102 in DCM.
Sensing the inductor current iL across inductor current sense resistor 124 results in power losses equal to iL2R, and “R” is the resistance value of inductor current sense resistor 124. Generally the value of “R” is chosen so that the losses associated with sensing the inductor current across inductor current sense resistor 124 are at least approximately 0.5-1% loss in total efficiency. However, when operating at above 90% efficiency, a 1% energy loss represents at least 10% of the losses. Additionally, controller 110 includes two extra terminals 130 and 132 to respectively sense inductor current sense voltage ViL—sense and signal FLYBACK. Extra terminals for an integrated circuit embodiment of controller 110 add extra cost to controller 110.